General suppliers

Cole-Partner Instr. Co.,

7425 N. Oak Park Avn., Chicago, Ill. 60648, USA

Edwards High Vacuum,

Manor Royal, Crawley, West Sussex, RH10 2LW, England.

Genevac Ltd.,

Alpha Works, White House Road, Ipswich, IP1 5LU, England.

Leybold-Heraeus,

Bonner Str. 498, D-5000 Koeln, FRG

Veeco Instruments Ltd.,

Abbey Barn Road, High Wycombe, Bucks., HP11 1RW, England.

Special equipment

Glassware, including PTFE taps

Bibby Science Products Ltd.,

Tilling Drive, Walton Industrial Estate, Stone, Staffs. ST15 OSA,

England. Tel (9782) 812121, Telex 36225, FAX 0785–813748.

J. Young {Scientific Glassware) Ltd.,

11 Colville Road, Acton, London, W3 8BS, England. Tel (01) 992 0891.

Glove boxes

Faircrest Engineering Ltd

4 Union Road, Croydon, CRO 2XX, England. Tel (01) 689 8741.

Tempo Trading Ltd.,

6 Rising Sun Industrial Estate, Blaina, Gwent, NP3 3JW, UK

Metal valves, connectors, etc.

Cajon Company,

9760 Shepard Road, Macedonia, Ohio 44056, USA

Hoke Inc.,

1 Tenakill Park, Cresskill, N.J. 07626, USA.

Nupro Company,

4800 East 345th Street, Willoughby, Ohio 44094, USA.

Swagelok Crawford Fitting Company,

29500 Solon Road, Solon, Ohio 44139, USA

Veeco,

Terminal Drive, Plainview, New York 11803, USA

Microbalances for weighing in vacuum

C.I. Electronics Limited,

Brunel Road, Churchfields, Salisbury, Wilts, England. Tel (0772) 336938,

The first words of this book are devoted to my friend Dr Steve Pask, who had urged me to undertake its writing in collaboration with him, which I eventually did. His contributions include much of the text and some valuable constructive criticism, and it was to the great regret of both of us that circumstances forced him to withdraw from the project. Without him this book would never have come to be. However, I alone take responsibility for what it is now.

Amongst my hitherto unacknowledged debts is one to H. A. Skinner, my PhD supervisor (under the general direction of Michael Polanyi), who introduced me to, amongst many other things, the arts of glass blowing and vacuum technique when I started at Manchester in 1944. He, in turn, had learnt them from H. W. Thompson at Oxford.

In the context of practical, especially vacuum line, chemistry, it is my pleasure to express a warm appreciation of Fred Fairbrother, a big man in every way. He had a reputation of sequestering scarce apparatus in his laboratory through a ‘non-return valve’, but towards me he always showed the greatest generosity in sharing equipment and his incomparable store of practical experience and theoretical insight.

It would be misleading, when writing about the Manchester University Chemistry Department in the late 1940s, to omit the impact made upon our little community by that human ball-lightning, Michal Szwarc.

Fundamentals

At the beginning of this century, scientists generally believed that all adsorption phenomena were of the sort we have called physisorption. Some (unspecified) long range attractive force drew gas phase matter toward a solid and the increased concentration of the gaseous substance near the surface was thought to be analogous to the retention of the Earth's atmosphere by the gravitational field. The adsorbed layer was viewed as a ‘compressed vapor’ with little or no interaction with the atoms of the substrate. However, compelling experimental evidence soon accumulated that pointed to another, distinctly different, form of adsorption.

Langmuir (1916) introduced and extensively investigated the idea that there can exist strong, short range forces between adsorbates and a substrate. He regarded the arrangement of atoms at the surface of a solid as a sort of Chinese checkerboard that defines a specific density of potential adsorption sites. Foreign gas atoms that strike the surface may either bounce back into the gas phase or bind to one of these sites through formation of a surface chemical bond. The latter process is termed chemisorption and, in this view, it is not unreasonable to regard the adsorbate/substrate complex as an enormously large molecule.

Adsorption, and chemisorption in particular, lowers the free energy of any closed system that contains only a free surface and atoms or molecules in the gas phase.

Introduction

The preceding chapters have focused entirely on the equilibrium free energy state of an isolated clean crystal surface. Unfortunately, many of the most interesting conceptual (and commercial) issues in surface physics intimately involve the interaction of a solid surface with foreign matter. If the interaction is strong, it is necessary to treat the surface and the foreign material as a single combined system. This is the subject of Part 2 of this book. However, if the interaction is weak, the surface merely responds to the external perturbation while retaining its individual identity. In fact, any real experiment designed to probe the properties of even an isolated surface invariably perturbs the system and invokes a characteristic response. This response is determined by the low-lying excited states of the system.

For example, consider an experiment designed to determine the binding energy and dispersion of an electronic surface state. In practice, one uses photoemission spectroscopy to measure the kinetic energy and propagation vector of an electron ejected from the sample into the vacuum. However, what one actually measures is the energy and relative momentum of an excited electronic state (with a finite lifetime) that consists of 1023 electrons in the presence of the surface-localized hole left behind by the photoelectric event. A priori, this could be a horribly complex state of the interacting many-body system.

Introduction

A complete characterization of a solid surface requires knowledge of not only what atoms are present but where they are. Just as in the bulk, it is not that the atomic coordinates as such are of much direct interest. Rather, our concerns generally will center on electronic and magnetic properties and it is the geometrical arrangement of the surface atoms that largely determines the near-surface charge and spin density. Put another way, the nature of the surface chemical bond depends in detail on surface bond lengths and bond angles. The corresponding bulk structural issues normally are resolved by x-ray diffraction. Unfortunately, the extremely large penetration depth and mean free path of x-rays severely limits their routine use for surface crystallography. Consequently, much effort has been devoted to the invention and application of alternative experimental approaches to surface-specific structural analysis. Although a number of common techniques will be discussed below, it is a sobering fact that no single surface structural tool has emerged that can be used as easily and reliably as x-rays are used for the bulk.

Appeal to theory does not offer much relief. In principle, a solid adopts the crystal structure that minimizes its total energy. We know how to write down an exact expression for this energy; it is a parametric function of the exact position of all the ions in the material.

Introduction

Our penultimate chapter is concerned with the chemical physics of surface reactions. More precisely, the consistent microscopic perspective adopted throughout this book demands that we ask the following question: can the experimental and theoretical methods of surface physics provide a useful account of real-life surface reaction processes? Let us emphasize the word ‘useful’. It is one thing to construct a post facto analysis which faithfully reproduces some set of observations. It is quite another to formulate general principles which provide qualitative insight and lead to quantitative predictive power. The selected examples below are intended to demonstrate that this discipline is just now passing from the former perspective to the latter.

Surface reactions are complex events which come in many guises for many purposes. Often (but not always!) one begins with some combination of species in the gas phase: the reactants. In heterogeneous catalysis, the purpose of the surface is to confine the reactants to a two-dimensional space in order to increase the probability for collision and reaction. The sought-after reaction products desorb for collection with no material change in the surface itself. Compare this to metalorganic chemical vapor deposition (MOCVD) growth of compound semiconductors (Dupuis, 1984). Therein, the surface stimulates a decomposition reaction. Unwanted species desorb and the desired species incorporate themselves into the solid.

Introduction

The physics of chemisorption hinges on the static and dynamic properties of the surface chemical bond. Bond formation, bond stability and bond dissolution all are crucial to our subject. In this chapter we begin our investigation motivated by the well-known intimate connection between bonding and structure (see, e.g., O'Keeffe & Navrotsky (1981)). The spatial distribution, strength and reactivity of the electronic bonds within a chemisorption complex depend sensitively on the relative position of the adsorbate and substrate nuclei. In the best case, all bond distances and bond angles will be known to any desired numerical accuracy. Minimally, we should know the local symmetry of the adsorption site, the gross orientation of the adsorbate with respect to the surface and the nature of any structure within the adsorption layer itself. Unfortunately, in the world of surface science, none of these quantities reveals itself in a straightforward, mechanical fashion.

What is the geometrical arrangement of atoms in the surface region of a clean crystal after adsorption by a foreign species? Consider first the case of a single adatom. As discussed in Chapter 8 for the case of Au/NaCl(100), the most probable binding sites occur at substrate positions where the total adatom/substrate potential energy of interaction has minima.

The present volume is a graduate-level introduction to the physics of solid surfaces. It is designed for students of physics, physical chemistry and materials science who are comfortable with modern condensed matter science at the level of, say, Solid State Physics by Ashcroft & Mermin (1976) or Principles of the Theory of Solids by Ziman (1972). In the latter, Ziman points out that scientific knowledge passes from the laboratory to the classroom by a sequence of literary vehicles: original research papers, review articles, monographs and finally textbooks. I believe this book fits well into none of these categories. It is not a textbook – at least not in the traditional sense. The field of surface physics is simply not mature enough to support such an enterprise; too many results are untidy and too many loose ends remain. On the other hand, it is not a review or monograph either. My purpose is neither to set down an established wisdom nor to establish priority among claimants. Indeed, I steadfastly ignore who did what when – except when it is a matter of historical interest.